1. Field of the Invention
The preset invention relates to a sensor control device for controlling a sensor having a diffusion chamber, an electromotive force cell and a pump cell.
2. Description of the Related Art
Known sensors for linearly detecting oxygen over a wide concentration range within exhaust gas exhausted from an internal combustion engine include a diffusion chamber into which a gas to be measured (exhaust gas) flows through a diffusion rate controlling layer, and two cells, that is, an electromotive force cell (oxygen concentration measurement cell) provided with electrodes on a solid electrolyte layer and a pump cell (oxygen pump cell).
In order to maintain a detection voltage (Vs voltage) outputted from the electromotive force cell of this sensor to a predetermined value (target voltage value), a sensor control device is provided which is configured to perform feedback control of pump current supplied to the pump cell to thereby control the sensor.
However, a case may arise in that feedback control is not suitably performed due to factors such as variation in a characteristic of the sensor or use environment, and hence the pump current value becomes unsuitable. As a result, the detection voltage of the electromotive force cell does not converge to the target voltage value to thereby cause oscillation in the sensor control device performing feedback control.
In order to address this problem, a technique is known in which, for example, the inner side electrode of a pump cell and the detection electrode of an electromotive force cell are formed as a common electrode, and a leakage resistor having a small resistance value is added to the common electrode (see Patent Literature 1). Further, another technique is known in which a feedback resistor is added in parallel to the outer side electrode of a pump cell and the cell electrode of an electromotive force cell, and a voltage having the same phase as that of a pump current is superimposed on a Vs voltage to thereby suppress oscillation (see Patent Literature 2).
Furthermore, still another technique has been proposed in which a high-frequency component part of a pump current, obtained by eliminating low frequency components of a predetermined cut-off frequency or less from a pump current, is caused to flow to an electromotive force cell, thereby suppressing oscillation of a control circuit connected to a sensor (see Patent Literature 3).
[Patent Literature 1] Japanese Patent No. 2624704
[Patent Literature 2] JP-A-5-256817
[Patent Literature 3] JP-A-2002-243700